I use Kamar Varnish by Krylon® to seal all my paintings. Normally, I apply two separate thin coats (waiting until dry between the coats) to my paintings. Along with great protection, I find it gives the paintings a slightly glossy look that really enhances the colors and details of the painting.
You can apply the varnish to acrylic and oil paintings as soon as they have dried to the point they are no longer tacky.
I use Krylon® Workable Fixitif to seal my drawings.